Jump to content

Postwick Tide Gauge


Delta

Recommended Posts

This can also be done using Mils. These are used by artillery, naval gunnery and long range rifle shooting.

One Mil is the angle subtended by one metre at 1000 metres.

Get your GPS APP and position yourself 500 metres down (or up) the river from the bridge, then look at it through a telescopic rifle sight marked in Mil dots.

If the angle at that range is just over 21 Mils, then the bridge height is 10.6 metres, or 35 feet.

Link to comment
Share on other sites

If you use water bags, they cannot sink, so once in the river will limit your maximum heel. 

You could get one of those flying camera things, then position it a few hundred yards behind the boat looking at the bridge, using parallax, you can see if you have enough clearance, but if all fails, save the video, and get £250 from "You've been framed" lol.

  • Like 1
Link to comment
Share on other sites

I was thinking about this, tie the water bag, or heavy weights to the boom, and push the boom out, solves the problem of getting the initial heel as well lol.

 

Using the sine rule, if you heel 45 degrees, your mast height will have been shortened to around 71 % of it's height.

Link to comment
Share on other sites

1 hour ago, Viking23 said:

Using the sine rule, if you heel 45 degrees, your mast height will have been shortened to around 71 % of it's height.

I should think you will also fill the cockpit up with water, at that angle. There is also the thought that most if not all of the rudder will be out of the water, so no control.

If (repeat if) I was going to do this, I would have a cruiser close abeam, and anchor the end of the jib halyard to it. You can then careen the yacht by hauling in on the halyard winch. good use of the speed of both boats' engines would keep everything straight as you go under.

This would require seamanship of a high order, and I still wouldn't want to try it myself!

 

  • Like 1
Link to comment
Share on other sites

  • 2 weeks later...

45 degrees was an illustration not a practical proposition and my rudder would not be out of the water the 71& is not 71% shorter it would be 71% of the height of the mast but no matter.

Anyway, I now have the use of a laser measure now so I just need a lift to get me under the bridge or I may have to get my old slalom kayak out again... or deflate the dinghy and launch nearby but it does not have a licence these days so no go.

Link to comment
Share on other sites

The laser measure might not differentiate between the girders and the underside of the bridge, so be aware of that, so take many measurements to see if you can measure both for reference. 

Re boats, I would think someone might be around to help with this trial.

 

 

Link to comment
Share on other sites

with a laser measure you can position the red dot wherever you want, I have used them to measure the conductor height of overhead power lines, the only tricky bit there is getting the dot onto the wire for long enough, in that application anything over 10m becomes tricky if there is any wind. for measuring to a flat beam, it should have no problems.

dont forget to add the height of the actual measure, plus the height between it and the water surface. mind you with the measure it should also be possible to get the height of the mast, if there is an almost horizontal surface to spot the dot onto at the right height.

Link to comment
Share on other sites

Some of the lower cost measuring devices, use a laser led to target, but an ultrasonic device to measure, ok for walls, ceilings etc the ultrasonic device is not pin point accurate, as it has a wide spread, and I doubt that many would be aligned with the laser led, so make sure you choose your technology carefully.

Link to comment
Share on other sites

Yes, thanks for that, it will/may be interesting to see what results I can get, as the laser beams form a circleof dots on the target and so some will fall on and some between any projections but should be esily visible around dusk.  Realistically, I am only interested in how close to 35 or so feet it is, in the middle, so as to ascertain wehether or not there is any safety margin or whether the published height is actually given as the lowest point on the span which would be at the supports at each end.  I have probably discounted attempting to travel with my masts fixed now but the question will not go away until I have had at least a good attempt at checking the height for myself.  A single laser beam could be more accurate but I am not aware if there is a convenient source of such a calibrated item outside the military, scientific or surveying communities.  It would not be likely to be affordable anyway.

Link to comment
Share on other sites

depending upon what you class as affordable, -

http://www.laser-measure.co.uk/leica-disto/leica-disto-d2.html

this is what I would call a decent laser measure - with 1.5mm accuracy. it was something similar to this that I trialed on overhead line heights, so at 10m it could pick up a 15mm wide target.

This is very much bottom end of the survey laser measure market with only a 60m max range, though the range can be extended using retro reflectors (these reflect the beam at the same angle it comes in, so extend the capabilities of the measures - but do rely on you being able to place the reflector on the target.

I suppose affordable depends upon how much a new mast could cost if you got it wrong?

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Sponsors

    Norfolk Broads Network is run by volunteers - You can help us run it by making a donation

×
×
  • Create New...

Important Information

For details of our Guidelines, please take a look at the Terms of Use here.